111 
is really no well defined boundary line, but the transition zone 
between the two regions. Different authors have fixed the 
timber-line differently. 
rest Line, that is where the continuous forest stops. 
2. The ae aa where the tree ceases to form communities 
of larger or smaller s 
ree Line, ane i arboreal species cease to form trees. 
4. Absolute Timber-line, that is where these species disappear 
altogether even as “krumho 
is superfluous to Bae more than two of these lines, the 
a line and the absolute timber-line. The region between 
ese two is a transition zone, or better, a zone of strife. 
continuous warfare goes on between the forest and the alpine 
grassla 
Then fe treated the factors that are governing the timber-line. 
The conditions that have been given as causing or modifying the 
aa lin 
. The es temperature during the growing seaso I 
re Rockies there are no records of the lowest eee that 
the trees can stand, but in general the arborescent flora dis- 
appears where the mean peer wale during the warmest month 
of the year does not rea oC 
2. Short growing season. In the arctic region this factor acts 
parallel to the preceding, but not so in the alpine region 
heads of the valleys where the snow drifts are formed during the 
winter and melt late in the summer and along glaciers and perma- 
nent snow, the frost is kept longer in the ground and the growing 
season is naturally shortened. Therefore, the timber-line is 
much lower in the valley heads than on the slopes. 
. Late frost on at of lack of aes ays from the snow 
oe frost often kills large tracts of conifers which alot 
produce a second crop of shoots in the same season. 
. Strong desiccating winds. This ae is much more 
important an usually estimated. The trees of the timber-line 
and especially those few isolated stragglers above the real forest- 
line show great effect from the wind. The trees are not only low, 
stunted, ragged, with enormously digngated branches spreading 
